I feel dizzy when lying down?

As the title says, when i lie down or sit down I feel a bit dizzy and I don't feel very good. I have to close my eyes when it happens. I also feel like puking when I feel dizzy. When i do sit down or lie down and I move my head around it takes a while but i will get dizzy after a few minutes. I don't really understand but please help me. If you need more information in order to help me find why this is happening feel free. Thanks in advance.

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